1. Field of the Invention
The present invention relates to a disk recording medium such as a compact disk (CD) or a digital versatile disk (DVD) in which data is recorded in high-density, and method and apparatus for reproducing data recorded in the high-density disk recording medium.
2. Description of the Related Art
FIG. 1 is a partial block diagram of a conventional optical disk device for reproducing data from a disk. The disk device of FIG. 1 comprises an optical pickup 2 for detecting recorded signals from an optical disk 1 such as a CD or a DVD, a signal detector 3 for yielding a reproduced RF signal through adding the signal components detected from the disk 1, a comparator 4 for converting the reproduced RF signal into a binary signal through triggering the RF signal with a reference zero level, a phase lock loop 5 (PLL) for synchronizing an internal reference clock with the binary signal, a bit stream generator 6 for generating a bit stream according to level and its length of the binary signal using the synchronized reference clock, and a demodulator 7 for restoring original digital data from the bit stream through correcting error, if any, with error correction code contained in the bit stream.
The conventional data reproducing operation conducted by the optical device of FIG. 1 is described.
The optical pickup 2 makes a laser beam be incident onto mark or pit trains formed along a track of a recording surface of the disk 1 as shown in FIG. 2, and converts the reflected beam from the beam spot into an electrical signal. The beam reflected from a mark or a pit is converted into a low-level electrical signal, whereas the reflected one from a space between marks or pits is converted into a high-level signal.
The components of the converted electrical signal are added in the signal detector 3 and then results in a RF reproduced signal which is applied to the comparator 4. The comparator 4 converts the RF reproduced signal into a binary signal which has only two levels through triggering the RF signal above and below a reference slicing level, and applies the binary signal to the PLL 5 which synchronizes an internal reference clock with the binary signal in phase. For example, the PLL 5 adjusts the clock speed of a 4.3218 MHz internal reference clock to have the falling edge of the internal clock coincided with that of the reproduced binary signal if the disk 1 is a compact disk. Then, the PLL 5 applies the synchronized reference clock and the binary signal to the bit stream generator 6.
The bit stream generator 6 converts the binary signal corresponding to marks and spaces formed in the disk 1 into bit stream referring to the synchronized reference clock, and the demodulator 7 restores the bit stream into original data while correcting the restored data based on the channel correction code contained in the bit stream.
To be brief, electrical signal components detected from a general optical disk by the pickup 2 are added into a reproduced RF signal which is converted into a binary signal after being compared with a reference slicing level, and then is converted into a bit stream based on a reference clock synchronized with the binary signal in phase.
Therefore, the minimum length of a mark or a pit to be formed in an optical disk should be long so that a RF curve generated corresponding to the mark or pit might cross the reference slicing level to be triggered without jitter.
In case that an optical disk is a CD for which the reference clock is 4.3218 MHz, the minimum length of a pit or a space is specified to three pulses, i.e., 3T, which corresponds to 0.86677 μm, among related companies so that an RF curve produced from the shortest pit formed in the CD might be converted into a bit stream normally.
In the meantime, the related companies are developing a technology to increase the recording capacity of an optical disk. The technology under development is to use a blue laser beam of which wavelength is shorter than that of conventional laser beam, or is to shorten the marginal gap, which is called as ‘pitch’, between two tracks.
However, since the requirement that a RF curve generated corresponding to a minimum-length mark or pit have enough transition duration to cross the reference slicing level without jitter should be still satisfied, the shortest mark or pit is not inevitably less than 3T, which puts restriction on increasing recording capacity of a track.